| 1. |
- Hallberg, Håkan, et al.
(författare)
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Model Describing Material-Dependent Deformation Behavior in High-Velocity Metal Forming Processes
- 2009
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Ingår i: Journal of Engineering Mechanics - ASCE. - 1943-7889 .- 0733-9399. ; 135:4, s. 345-357
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Tidskriftsartikel (refereegranskat)abstract
- A constitutive model for rate-dependent and thermomechanically coupled plasticity at finite strains is presented. The plasticity model is based on a J(2) model and rate-dependent behavior is included by use of a Perzyna-type formulation. Adiabatic heating effects are handled in a consistent way and not, as is a common assumption, through a constant conversion of the internal work rate into rate of heating. The conversion factor is instead derived from thermodynamic considerations. The stored energy is assumed to be a function of a single internal variable which differs from the effective plastic strain. This allows a thermodynamically consistent formulation to be obtained which, as shown, can be calibrated by use of simple procedures. Choosing 100Cr6 steel in two differently heat treated conditions as prototype material, experimental tests are performed, enabling the model to be calibrated. Significant differences in deformation behavior are noted as the differently heat treated specimens are compared. In addition, the local stress-updating procedure is reduced to a single scalar equation, permitting a very efficient numerical implementation of the model. The constitutive formulation proposed was employed in an explicit finite element solver, illustrative simulations of a high-velocity metal forming process being performed to demonstrate the capabilities of the model and certain characteristic traits of the materials that were studied.
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| 2. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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Electron microscopy of white-etching band generated by high-velocity parting-off of 100CrMn6 steel
- 2008
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Ingår i: Materials Science & Engineering A: Structural Materials: Properties, Microstructure and Processing. - : Elsevier BV. - 0921-5093. ; 480:1-2, s. 489-495
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Tidskriftsartikel (refereegranskat)abstract
- High-velocity parting-off has been applied to 80 mm bars of pearlitic 100CrMn6, resulting in shear localisation and white-etching bands in a severely deformed region below the fracture surface. Electron microscopy showed that going from the bulk material towards the fracture surface the grains become elongated and refined. The region below the fracture surface can be divided into three subzones: 50–100 μm below the surface grains are elongated, cementite lamellae are distorted, break up and the lamellar spacing decreases.
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| 3. |
- Ryttberg, Kristina, 1977
(författare)
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Fracture behaviour and related microstructures of tool steels parted-off at high rates of strain
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off, or adiabatic cutting, has shown to result in very low material waste as well as narrow dimensional tolerances of parted-off samples. However, for optimisation of the process improved knowledge is required regarding fracture characteristics and deformation mechanisms associated with the process. For evaluation of samples cut by commercially available technology, incorporating impact velocities of 5-10 m/s, a hydraulic high-velocity pressing machine with a parting-off tool was used. In addition, for evaluation of the possible influence of a higher impact velocity on the shear localisation, a non-conventional method was developed. In that case velocities up to 285 m/s could be employed. By measurement of velocities an estimation of fracture energies could be made. Microstructure evolution during deformation and fracture was characterised using optical microscopy combined with scanning and transmission electron microscopy.It has been concluded that for impact velocities of 5-285 m/s, the parting-off is, within the full range of velocities, initiated through shearing resulting in ductile shear fracture. However, depending on sample size and velocity the fracture mechanisms active in some parts of the fracture are ductile tensile indicating a triaxial stress state. Microstructural studies showed that the depth of the severely deformed region below the fracture surface is more dependent on the size of the sample than on the microstructure, resulting in much smaller deformed regions for smaller samples. The severe deformation results in cracks: For small samples only microcracks initiated on precipitates such as carbides and MnS inclusions, but for larger samples inter- and intragranular cracking also occurred. Electron microscopy of the severely deformed region below the fracture surface has shown three different types of structures: Right below the fracture surface a white-etching band (appears white in optical microscopy) consisting of nanocrystalline equiaxed grains was occasionally found. The grain size within this region was between 50 and 150 nm. The region below the white-etching band consisted of a mixture of equiaxed grains and highly elongated subgrains. The third region, located below region two, is composed of highly elongated subgrains. In the case of speroidise-annealed 100Cr6 parted-off at an impact velocity of 225 m/s the elongated subgrains were shown to have a mutual orientation, adjacent subgrains having (110) type of planes parallel. This could be an indication of formation of white etching bands being a mechanically rate controlled process involving dynamic recovery. In the case of parting-off pearlitic 100CrMn6 steel it has been shown that the lamellar spacing of the cementite decreases and cementite lamellae aligned in a direction perpendicular to the generated fracture surface are severely distorted during the extreme deformation, resulting in breaking up of the lamellar structure. The white-etching band consists of small finely distributed carbides in a ferrite matrix.
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| 4. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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High strain rate deformation induced by high-velocity forming of 100Cr6 steel
- 2008
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Ingår i: proceedings of The 9th International Conference on Technology of Plasticity (ICTP), Gyeongju, Korea, September 7-11, 2008. - 9788957081525 ; , s. On CD or www.ictp2008.org-
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Konferensbidrag (refereegranskat)abstract
- This paper concerns the development of the near-net-shape manufacturing technique of high-velocity forming. The microstructural response to the high strain-rate deformation was evaluated for 100Cr6 steel in two differently heat treated conditions: spheroidise annealed (hardness 250 HV) and quenched and tempered (330 HV). A high-velocity pressing machine was used to conduct the forming tests on cylindrical specimens. By applying different impact velocities on samples with varying sizes deformation mechanisms could be related to strain and strain rate. Extreme localization of deformation resulting in white etching bands was seen in quenched and tempered samples but not in spheroidise annealed samples. Furthermore, the strain and strain rates were shown to affect the tendency for extreme localization. No white etching bands were found in samples strained to less than 1. Appearance of white etching bands could be seen in the macroscopic shape of the samples where the more homogeneously deformed samples showed a typical barrel shape while the strongly localized samples were slightly more bell shaped. It can be concluded that for high-velocity forming to be applicable to 100Cr6 steel a careful choice of the combination strain/strain rate needs to be made.
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| 5. |
- Ryttberg, Kristina, 1977
(författare)
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Microstructural changes in high strength steels exposed to large deformation and high strain rates
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Increasing steel prices and environmental aspects have put forward the demand to reduce material consumption and energy usage in manufacturing industry and near-net-shape manufacturing techniques have thus become increasingly important. High-velocity parting-off,high-velocity forming and cold ring rolling are three such manufacturing methods that exhibit great potentials in terms of material waste reduction. However, all three processes involve large degrees of deformation that is not homogeneously distributed in the samples and the scientific knowledge regarding deformation mechanisms active in these processes is low. In order to allow for process optimisations a thorough understanding of associated deformation behaviour and microstructural changes is needed. Three steels have been used in this work: two bearing steels and one carbon steel.Through studies employing high-velocity parting-off incorporating impact velocities of 5-285 m/s, the fracture mechanisms active during material separation was shown to be a mix of ductile shear and ductile tensile fracture and to some extent failure by adiabatic shear banding. High-velocity forming tests were then conducted to evaluate parameters controlling the strain localisation and initiation of adiabatic shear bands (ASBs). Strain and strain rate were shown to be important for strain localisation. However, most important was shown to be the microstructure coupled to hardness where quenched and tempered samples developed ASBs while spheroidise annealed samples did not. By using electron microscopy the microstructure in the ASB regions generated by parting-off and forming were compared and shown to be composed of three types of structures: Within the ASB nanocrystalline equiaxed grains with a size of 50-150 nm were found, while adjacent to the ASB the microstructure consisted of a mixture of equiaxed grains and highly elongated subgrains. Outside this region only highly elongated subgrains were found. The elongated subgrains were shown to have a mutual orientation, adjacent subgrains having {110} type of planes parallel. This could indicate that formation of ASBs is a mechanically rate controlled process. In addition it was shown that smaller carbides were dissoluted while larger spheroid carbides remained and possibly also facilitated the refinement of microstructure in the formation of ASBs.Cold ring rolling tests were done to further investigate the effect of large deformations on microstructure and texture. The plastic deformation was shown to be most severe near theinner diameter of the rings decreasing towards the area of the outer diameter. By employing electron back scatter diffraction the ring rolling process was shown to result in a combined fibre texture where was parallel to the rolling direction and was parallel to the ring radial direction in the centre of the ring, indicating more or less plain strain in this region. Below the inner diameter the texture was of the {110} type indicating contribution of shear.
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| 6. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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Microstructural evolution during fracture induced by high strain rate deformation of 100Cr6 steel
- 2009
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Ingår i: Journal of Materials Processing Technology. - : Elsevier BV. - 0924-0136. ; 209:7, s. 3325-3334
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the influence on parting-off mechanisms of a significantly higher impact velocity compared to commercial methods. A specially developed method, allowing for parting-off velocities ranging from 38 to 285 m/s has been applied to 100Cr6 steel bars. Two different heat treatments; spheroidise annealing (SA) and quench and tempering (QT) were employed to produce two different microstructures of a hardness of 255 and 310HV, respectively. It appears that the failure mechanisms active in the current process are virtually identical to failure occurring during high-velocity parting-off using commercially available machines, which displays shear fracture and adiabatic shear banding. It was also concluded that the estimated energy consumed during parting-off is not dependent on microstructure. Prior to parting-off, impact results in shear localisation, which is somewhat increased with increased impact speed. Furthermore, shear strains at fracture also increaseswith impact speed, from a value of about 2.2 at the lowest impact velocity to almost 3.5 at an impact velocity of 285 m/s. The heavy deformation causes a grain refinement. Right below the fracture surface three subzones can be found in the microstructure; a white etching band (WEB) (only present on some locations), equiaxed grains and then elongated subgrains. The grain size within these zones varies between 50 and 150 nm. The findings of elongated subgrains of a mutual orientation, adjacent subgrains having {110} type of planes in parallel, support the theory of formation of white etching bands being a mechanically rate controlled process.
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| 7. |
- Ryttberg, Kristina, 1977, et al.
(författare)
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Microstructure and texture development during ring rolling of 100Cr6
- 2008
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Ingår i: Ceramic Transactions. - 1042-1122. - 9781574982961 ; 200, s. 301-309
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Konferensbidrag (refereegranskat)abstract
- Pre-turned 100Cr6 steel rings with spheroidised annealed microstructure have been cold ring rolled to two levels: expanding the outer diameter by 10 and 20%. The final microstructure was shown to be varying throughout the cross-section of the ring, where the largest deformation was found close to the surface of the inner diameter. This was reflected in the hardness profile showing the highest hardness values close to the inner surface. Scanning electron microscopy showed that the microstructure below the surface of the outer diameter consisted of a greatly refined ferrite. Close to the surface of the inner diameter the structure was even more refined. Electron back scatter diffraction together with X-ray diffractometry showed that cold ring rolling changed the existing texture of the ring blanks, where {110}-planes were parallel to the axial direction of the rings, towards {110}-planes becoming parallel to the rolling direction. This resulted in a weak {110}-texture. Close to the surface of the outer diameter the greatly refined ferrite had decomposed into a more random orientation distribution.
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| 8. |
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